In recent years, electric vehicles such as an electric automotive vehicle having a motor as a drive source, and a hybrid automotive vehicle having an engine and a motor as a drive source have come into use. In the electric vehicles, there is widely used a system of converting DC power supplied from a rechargeable secondary battery (battery) mounted on a vehicle into AC power such as three-phase AC power by an inverter, and supplying the inverted AC power to the motor for driving the vehicle. The inverter is configured to covert DC power to AC power by turning a plurality of switching elements ON and OFF. In many cases, a synchronous electric motor or a synchronous motor generator is used as the motor for driving the vehicle.
In the electric vehicle in which the battery, the inverter, and the synchronous motor are used, there is a case where the vehicle stops even though an accelerator pedal is pressed. As an example, on a sloping road, it is a state in which the vehicle does not travel to climb the slope but stops halfway up the sloping road even though a driver presses the accelerator pedal downward. In this case, since the number of rotations of the synchronous motor is zero even though a current flows to the synchronous motor, one of the switching elements of respective phases U, V, W of the inverter is in an ON state, and other switching elements stay in an OFF state (such a state is expressed by an expression “the inverter is in the locked state”). Therefore, a current intensively flows toward the switching element in the ON state, which causes an increase in temperature of the switching element.
Accordingly, there is proposed a method of avoiding a specific switching element being in the ON state, by changing a phase angle of the current when the motor is bought into a locked state, and restraining a temperature increase in the switching element (For example, see JP-A-2005-354785).
Since the rotational angle of the synchronous motor is the same as a change in phase angle of the current, if the phase angle of the current is changed irrespective of an operation of the acceleration pedal by a driver as described in JP-A-2005-354785, the angle of rotation of the synchronous motor changes irrespective of the operation of the accelerator pedal by the driver, and the electric vehicle swings slightly in the fore-and-aft direction. Therefore, there arises a problem of deterioration in drivability.
There is a method of lowering a current of the switching element when the inverter is brought into the locked state. In this case, however, an output torque from the motor is lowered. Therefore, as in the example described above, there occurs such an event that the vehicle slips down on a sloping road in a state in which the vehicle does not travel to climb the sloping road, but stops halfway up the sloping road. In this case as well, there arises a problem of deterioration in drivability.